They don't just get tons of action... They get it balanced.
When lines come out, the Sharps move in on their overnight limits. I doubt Pinnacle is the one actually moving the lines.
Consider the line Dog +7 -105 vs Fav -7 -105. If the dog is the "right" side, Pinnie's line will be Dog +7 -112 Fav +7 +102 by the next morning. so then the rest of your public bettors see Fav +7 +102, or Fav +7 -110 everywhere else. So the rest of the week, the public and sharps take turns bumping the line a few cents. And the entire time, Pinnie's line is the best on both sides!
Pinnacle is the ultimate funneling machine - its reduced juice draws public money to offset sharp money. And due to the way they shift their lines (and that they want balanced action, even against public money), they don't care if sharps play there. Ever heard of a wise guy getting kicked out of there?